3.3 PHASE CHANGES
VOCABULARY:
phase change, endothermic, heat of fusion, exothermic,
vaporization, heat of vaporization, evaporation, vapor
pressure, condensation, sublimation, deposition
OBJECTIVES:
n  Describe phase changes.
n  Explain how temperature can be used to recognize a
phase change.
n  Explain what happens to the motion, arrangement, and
average kinetic energy of water molecules during
phase changes.
n  Describe each of the six phase changes.
n  Identify phase changes as endothermic or exothermic.
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STATES OF MATTER:
PHASE CHANGES
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PHASE CHANGES
phase change - reversible physical change that
occurs when a substance changes from one
state of matter to another
n 
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Six common phase changes are:
1. Melting
4. Condensation
2. Freezing
5. Sublimation
3. Vaporization
6. Deposition
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TEMPERATURE &
PHASE CHANGES
Temperature of a substance does NOT change
during a phase change.
page 85 in textbook
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TEMPERATURE &
PHASE CHANGES
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TEMPERATURE
&
PHASE
CHANGES
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ENERGY & PHASE CHANGES
energy is
transferred between
substance & its
surroundings
Energy is either
absorbed or
released during a
phase change.
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ENERGY & PHASE CHANGES:
ENDOTHERMIC
endo - inside
endothermic change - system absorbs energy
from its surroundings
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MELTING (ENDOTHERMIC)
heat flows from air to ice à ice gains more
energy à molecules vibrate more quickly
v  @ melting point (0°C), some molecules gain
enough energy to overcome attractions & move
from fixed positions
v  melting is complete when all molecules have
enough energy to move à after phase change,
any additional energy gained will increase
average kinetic energy of molecules &
temperature will rise
v 
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FREEZING (EXOTHERMIC)
v 
v 
v 
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energy flows from liquid water to air à water cools
down à average kinetic energy of molecules
decreases à molecules move more slowly
@ freezing point of water (0°C), some molecules move
slowly enough for attraction between molecules to
have an effect
when all molecules have been drawn into an orderly
arrangement, freezing is complete à after phase
change, any energy removed from ice decreases
average kinetic energy of molecules, & temperature of
ice drops
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ENERGY & PHASE CHANGES:
ENDOTHERMIC
heat of fusion - energy a substance must absorb
in order to change from solid to liquid
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v 
varies from substance to substance
v 
e.g. heat of fusion for water: 1 gram (g) of ice
absorbs 334 joules (J) of energy as it melts
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ENERGY & PHASE CHANGES:
EXOTHERMIC
exo - outside
exothermic change - system releases energy
to its surroundings
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how to know if energy is being
absorbed or released during
each change:
Think about how much energy you are expending for each hand
motion (mimicking particle movement).
Solid
Liquid
least energy
Gas
most energy
If you go from a motion that takes less energy, to one that takes
more energy, than energy is being absorbed (endothermic).
If you go from one hand motion that takes more energy, to one
that takes less, than the energy is being released (exothermic)
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TEMPERATURE &
PHASE CHANGES
GAS
endothermic
change
increase
temperature
(gain energy)
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LIQUID
SOLID
exothermic
change
decrease
temperature
(lose energy)
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MELTING & FREEZING
Arrangement of molecules in water becomes
less orderly as water melts and more orderly
as water freezes.
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VAPORIZATION
heat of vaporization - energy a substance must
absorb in order to change from a liquid to a gas
v 
varies from substance to substance
v 
v 
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e.g. heat of vaporization for water: 1 gram (g)
of water absorbs 2261 joules (J) of energy
when it vaporizes
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VAPORIZATION
vaporization: liquid à gas
vapor pressure - pressure caused by collisions
of particles in a vapor with walls of a
container. Increases as temperature increases
2 vaporization processes:
1. evaporation
2. boiling
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VAPORIZATION:
EVAPORATION
evaporation (liquid to a gas)
v  where it occurs: surface of liquid
v  temperature: below substance s boiling point
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VAPORIZATION: BOILING
boiling (liquid to a gas)
v  where it occurs: throughout liquid
v  temperature: at boiling point of liquid
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VAPORIZATION: BOILING
v  occurs
when vapor pressure = atmospheric
pressure
v  temperature
increases à water molecules
speed up à reaches B.P., molecules below
surface of liquid have enough kinetic energy
to overcome attraction of other molecules à
vapor à since vapor less dense than
surrounding liquid, bubbles rise to surface
à release water vapor into air
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Endothermic
(absorbs energy)
Exothermic
(releases energy)
Endothermic
(absorbs energy)
Exothermic
(releases energy)
FREEZING
MELTING
VAPORIZATION:
EVAPORATION
Phase
Change
Change of
State
Gains or Loses
Energy?
Endothermic or
Exothermic?
Melting
solid to liquid
gains
endothermic
Freezing
liquid to solid
loses
exothermic
Vaporization
liquid to gas
gains
endothermic
Condensation gas to liquid
loses
exothermic
Sublimation
solid to gas
gains
endothermic
Deposition
gas to solid
loses
exothermic
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THE PHASE CHANGES:
EXOTHERMIC & ENDOTHERMIC
Deposition
Freezing
Exothermic
Phase
Changes
Condensation
Melting
Enothermic
Vaporization
Sublimation
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